Walnuts

• Irrigation Management of Walnut Trees with a Limited Water Supply

Walnut is a relatively high value crop and the planted acreage continues to expand principally in the Central Valley of California. This expansion continues as competition increases for California water resources for domestic, industrial, environmental, and agricultural uses. As a result, it is likely that some walnut producers will be faced with periods of managing orchards with limited water supplies, especially during years with below-average snowpack and rainfall.

This article examines: the different stages of crop growth and development in walnut; their sensitivity to water stress; and the prospects of minimizing damage to trees, the current crop, and subsequent crops when water is in short supply. Specifically, the collective experience from UC research on deficit irrigation in walnut will be described. Finally, in addition to deficit irrigation management, other considerations to help prepare for and endure periods of limited water supply will be discussed.

EARLY SEASON TREE AND CROP DEVELOPMENT

Several early season tree and nut growth processes may be affected by limited irrigation and rainfall. Root growth is typically the first tree growth process each season, initiating in mid-February, before the walnut tree breaks dormancy and leaves emerge. Root growth occurs at a rapid pace from mid-February through mid-April and then slows during the summer. Depending on the variety, location, and weather, most commercial varieties of walnut trees break dormancy and begin to leafout between mid-March and mid-April. Leafout is followed soon by flowering, the emergence of pollen-producing male flowers (catkins) and the female (pistillate) flowers that forms the nut after pollination. Flowering is typically completed by late April and is followed by rapid expansion and sizing of the fruit (hull and immature nut) along with rapid, shoot growth, especially visible at pruning cuts. By early June, the fruit has reached its full external size and much of the shoot growth that may develop into fruit bearing wood in subsequent years has been grown. Orchard water use during this time is low compared to during summer, reducing the potential to save water from an early season deficit irrigation strategy.

MID SEASON TREE AND CROP DEVELOPMENT.

Beginning in mid-June, there is a shift from shoot growth and external fruit sizing to shell and kernel development. First, shell development and hardening occurs, followed by kernel filling and development. Kernel weight begins to accrue in late June and early July and filling continues through August and into September, until the nut is physiologically mature as indicated by "packing tissue brown". Packing tissue brown occurs before hull split and is visible by cracking nuts in half, removing the kernels, and observing the color of the tissue surrounding the kernel inside the shell. "Creamy white" packing tissue indicates the nuts have not reached maturity and may still gain weight, whereas, brown packing tissue indicates the nuts are mature and kernel filling has ceased. Kernel color is also a consideration during mid season crop development. Excessive water stress can decrease nut size and quality. Shoot growth on young trees will continue into July and August given favorable water status. Also important, bud formation on growing shoots continues into July and August (bud formation happens from leaf out and is likely completed before July on mature trees) and stress during this period has the potential to affect tree fruitfulness in the following season. Since orchard water use during this period is relatively high, any potential to save water with deficit irrigation during these stages of tree and crop development would be most helpful when faced with a limited water supply.

LATE SEASON TREE AND CROP DEVELOPMENT.

For most commercial varieties, hull split follows closely after walnuts have reached physiological maturity and kernel filling has ceased in September. During hull split, sufficient but not excessive tree water status is necessary to prevent hull tights and related problems with walnut quality such as darkening kernel color and mold. Excessive water stress as well as lack of water have both been shown to lead to a decrease in relative crop value (Lampinen, 2004,Lampinen et al., 2008a), mostly due to darker kernel color. Once the crop is harvested, the walnuts are no longer the primary draw upon the carbohydrates produced from photosynthesis. Instead, the root system becomes the primary sink to prepare the trees for overwintering and next year's bud break. A second flush of root growth occurs in September and October. Controlled levels of deficit irrigation during September and October may help harden-off new walnut shoots and lessen the likelihood of green walnut shoots being injured by autumn frosts. However, if deficit irrigation is too extreme in September and October and carries through the dormant winter season, it can lead to winter die-back among the current year's shoot growth as well as larger, older wood in trees. By late season, when orchard water use is beginning to decline as the days shorten and the temperatures lessen, there may be some opportunity to save water if faced with a limited water supply.

WATER DEFICITS.

Sources of water available to walnut trees include soil-stored moisture from rainfall retained in the root zone, any in-season rainfall absorbed by the soil, and applied irrigation water. Mature walnut orchards grown in the Central Valley, either conventional or high density plantings, can use about 41 to 44 inches of water per acre in an average year of unrestricted water use. Water use, often referred to as evapotranspiration (ET), can be higher depending upon how vegetation is managed in the orchard middles.

Data from a network of nearly 100 California weather stations are available for calculating daily reference evapotranspiration values -called ETo - from which walnut ETc (crop evapotranspiration) estimates can be derived. This information is made available to growers by the CIMIS Program in the California Department of Water Resources at http://wwwcimis.water.ca.gov/cimis. Some newspapers and irrigation districts also provide ETo data. The daily ETo data is summed into weekly ETo, then multiplied by a land coverage factor, or crop coefficient (kc) specific to walnut orchards. The product equals the orchard water use:

ETo x kc = ETc

Crop coefficients for mature walnut orchards have been experimentally determined for various times through the growing season. An orchard is considered to be mature when about 60% or more of the orchard floor is shaded at midday. For more information on water use of immature orchards, see http://ucmanagedrought.ucdavis.edu/ETimmatureTrees.cfm. Table 1 shows the calculations for determining walnut orchard water use, in two-week periods, from leaf out to leaf drop using the CIMIS station # 70 located near Manteca, California. Further information on walnut crop coefficients is available at http://ucmanagedrought.ucdavis.edu/ET_table10.cfm and estimates of walnut ET for various areas of California is at http://ucmanagedrought.ucdavis.edu/ETwalnuts.cfm. Walnut ET is also available from some UC Cooperative Extension Farm Advisors. For an example, see: http://cetehama.ucdavis.edu/Agriculture/Weekly_Soil_Moisture_Loss_Reports.htm

Figure 1 provides an example from the 2002 season of water use for fully irrigated walnuts in the northern Sacramento Valley when herbicides are used to control vegetation in the tree rows and mowing is used to control vegetation in the middles. Supplying less water than the walnut trees can potentially use reduces soil water availability, resulting in tree water deficits, and reduces transpiration. Figure 1 also shows the water use pattern for a walnut orchard managed in 2002 in the Northern Sacramento Valley using "mild" and "moderate" levels of deficit irrigation.

Figure 1: Example of water use in 2002 for a fully irrigated walnut orchard in the Northern Sacramento Valley and examples of two different levels of deficit irrigation and their effects on water use.

MONITORING DEFICIT IRRIGATION.

One of the best ways to monitor a water deficit in trees is to use a portable pressure chamber to measure "tree stress" or "stem water potential" (tree water status). To use this technique leaves from representative trees are first covered with a foil bag while still on the tree. The foil bags need to remain on the leaves for at least 10 minutes before leaves (still inside the foil bag) are detached from the tree, one at a time, and the water potential is measured using a pressure chamber. The pressure chamber measures the amount of pressure needed to force water out of the leaf petiole, indicating the tree water status. Measurements are taken at mid afternoon when tree water stress is highest and more stable. For more information on measuring tree stress with a pressure chamber Refer to: http://fruitsandnuts.ucdavis.edu/General_Management/Irrigation_&_Frost_Protection.htm. Midday stem water potential depends on both air temperature and relative humidity, with more negative values (more tree stress) as temperature increases and humidity declines. Current estimates of these values for fully irrigated walnuts are presented in Table 2. If measurements show that trees are close to these values, then no benefit of additional irrigation would be expected, and if trees are above (less stressed) then these values then over-irrigation may be occurring.

Table 2. Current estimates of midday stem water potential for fully irrigated (non-stressed) walnut trees under various temperature and humidity conditions.

AirTemperature (F)

Air Relative Humidity (%)

10

20

30

40

50

60

60

-3.8

-3.7

-3.6

-3.5

-3.3

-3.2

70

-4.2

-4.1

-3.9

-3.7

-3.6

-3.4

80

-4.8

-4.6

-4.3

-4.1

-3.9

-3.7

90

-5.6

-5.2

-4.9

-4.6

-4.3

-4.0

100

-6.5

-6.1

-5.7

-5.3

-4.9

-4.5

110

-7.8

-7.3

-6.7

-6.2

-5.6

-5.0

EXPERIENCE WITH DEFICIT IRRIGATION.

Some of the earliest UC research with deficit irrigation in walnut trees was done by Brown et al. (1977). Frequently irrigated Serr walnut trees were compared to non-irrigated trees on a deep clay loam soil with high water holding capacity. Moisture stress decreased yield, nut size, and kernel quality. Fully irrigated trees resulted in 43 percent higher returns compared to non-irrigated trees. Although tree water status was not reported, it is likely tree stress exceeded the mild or moderate stress levels that might be recommended in a regulated deficit irrigation strategy.

From 1986 through 1991, regulated deficit irrigation was evaluated more extensively by Dave Goldhamer (see the References section for links to multiple reports) in the Fresno area on a hedgerow planting of Chico walnuts. This study compared fully irrigated walnuts to walnuts under two levels of deficit irrigation. The fully irrigated trees received applied water equal to full crop evapotranspiration (41 to 44 inches per acre ETc). Applied water was reduced by 33 and 66 percent in the respective deficit irrigated trees. The irrigation cutbacks were applied uniformly across all crop growth stages as a consistent percentage of the seasonal ETc pattern. After three years of deficit irrigation, cumulative dry in-shell yield was between 20 and 40 percent less than the fully irrigated trees depending upon the extent of the water deficit. Walnut size was reduced and percent off-grade increased, negatively affecting crop value. After the first year of deficit irrigation, the principle negative effect on the crop was reduced nut size and a minor reduction in dry-inshell yield. However, shoot growth was also reduced during the first year of deficit irrigation and negatively influenced fruitfulness the next year. Repeated years of deficit irrigation lead to a cumulative reduction in shoot growth and new fruit bearing wood, and ultimately reduced the size of the trees and number of nuts per tree. The study also demonstrated that after irrigation was returned to a level where the combination of applied water and stored soil moisture was equal to full ETc, yield losses associated with deficit irrigation could be recovered after two years. During year one of recovery, the primary response was rejuvenated shoot growth and a minor yield improvement from increased nut size. In year two, the additional shoot growth from the previous year of full irrigation resulted in larger trees with more fruit bearing wood and a return of higher number of nuts per tree.

From 2002 through 2007, regulated deficit irrigation in walnut was evaluated further (Fulton et al., 2002,Lampinen et al., 2003, and Lampinen et al., 2004,Buchner et al., 2008). Interest re-surfaced partly because information was not available for the dominant commercial walnut variety, Chandler, and partially because midday stem water potential determinations with a pressure chamber, a plant-based measurement technique for quantifying tree water status, was available for on-farm use to assist with irrigation decisions. The hypothesis was that specific stages of walnut growth and development could be safely targeted for water deficits without damaging the crop. Particularly, the kernel filling and post-harvest stages of tree and crop development were targeted for deficit irrigation. Midday stem water potential was monitored to control when and to what extent the water deficits were imposed. Parallel experiments were conducted in a younger, hedgerow Chandler orchard (9 to 14 year old) in the Northern Sacramento Valley and an older (20 to 25 year old), conventional spaced Chandler orchard near Stockton in the Northern San Joaquin Valley. Walnut tree and crop response to mild and moderate water deficits were compared to fully irrigated, low stress trees at each experiment. The general strategy for imposing the water deficits was to avoid a water deficit during early stages of tree and crop development. Irrigation was gradually reduced to impose mild or moderate water deficits and tree stress during mid season and became more pronounced during late season stages of tree and crop development. For the low stress trees, the stem water potential was generally maintained between -3 and -5 bars tension during the early season and -5 to -7 bars tension by late season. The combination of applied water and soil storage approached full ETc (41 to 44 inches per acre). For the mild stress trees stem water potential ranged from -3 to -5 bars tension during the early season but stress slowly increased to about -8 to -10 bars tension by late season. The level of irrigation cutback was about 30 to 35 percent of full ETc or about 12 to 14 inches less applied water for the mild stress trees. For the moderate stress trees, stem water potential ranged from -3 to -5 bars tension during early season growth and stress gradually increased to -12 to -14 bars tension by late season. The level of irrigation cutback was about 50 percent of full ETc or about 20 inches less applied water.

The younger, bearing orchard in the northern Sacramento Valley was more sensitive to deficit irrigation, partly due to shallower soils with lower water holding capacity. At the end of three consecutive years of deficit irrigation, dry in-shell yield in the mild and moderate stress trees were 26 and 40 percent lower, respectively, than the fully irrigated trees. Walnut quality and crop value was also reduced three of the four years. Crop payment was as much as 27 and 40 percent lower than the low stress trees for the mild and moderate stress trees, respectively. Notably, measured shoot growth of pruned limbs was not significantly different between the low and mild stress trees yet dry in-shell yield was much lower. Shoot growth was significantly less in the moderate stress trees compared to the low stress trees. Further investigation suggested that with the mild stress deficit irrigation strategy, new fruit bearing wood and tree size could be sustained but the fate of the buds on the new shoots was affected in the next year. Following a season of mild deficit irrigation, less buds opened, fewer buds were floral buds, and fewer flowers emerged per floral bud that did open. The net effect was a significant decline in fruiting number per tree even though tree size was relatively unaffected. From this experience, it was apparent that employing deficit irrigation on Chandler walnut even at mild levels is likely to affect production. In 2006 and 2007, the deficit irrigated trees were returned to full irrigation and a low tree stress condition. Like the Fresno experience in the 1980's, full yield potential and improved nut quality was restored after two years of full irrigation. For the mild stress treatment, where shoot growth and tree size was unaffected by the water deficit but bud formation appeared to be affected there was evidence that full yield potential could be recovered after one season of full irrigation.

The experience at the older, San Joaquin Valley site, was somewhat different. Dry-inshell yield was not affected significantly during the first two years of mild and moderate levels of deficit irrigation. After the third year of deficit irrigation, dry in-shell yield was 18 to 20 percent lower in the mild and moderate stress trees. There were instances when walnut quality appeared to decline under deficit irrigation (i.e. increased shrivel in 2003 and increased mold in 2004) but the negative impacts were not nearly as consistent as those observed in the Northern Sacramento Valley trial. Speculated reasons for the difference in response at the Stockton experiment include: a less severe imposition of stress due to a deeper alluvial soil with greater potential to provide stored water in the absence of irrigation. The imposition of the water deficit was more subtle; much larger trees with a wider conventional tree spacing with no pruning provided a greater abundance of bearing fruit wood, possibly requiring more time for the cumulative decrease in the number of nuts per tree and ultimately dry-inshell yield to occur. The older trees have more developed root and trunk system possibly supplying more stored carbohydrate reserves to sustain the various phases of tree and nut development. The Stockton trial was not continued beyond 2004.

EMPLOYING REGULATED DEFICIT IRRIGATION.

From the discussion of research experiences above, it is clear that devising and employing a regulated deficit irrigation strategy in walnut during times of water shortage has risks and it is likely that some degree of crop damage will occur depending upon the extent and duration of the water deficit and the specific orchard setting. Walnut doesn't appear to be as strong a candidate for successful regulated deficit irrigation as some other permanent crops such as almond, pistachio, prunes, and wine grapes. However, if a water shortage is severe enough, some possible considerations for imposing a regulated deficit irrigation strategy might include:

Begin by considering deficit irrigation from a ranch wide perspective. There is some evidence that older, conventionally spaced orchards with larger trees on deeper alluvial soils may be better candidates to tolerate deficit irrigation, so if you have a choice among different orchards start with them first. Deficit irrigating younger, developing orchards can slow down canopy development, impacting yields for many years.

Use a pressure chamber to monitor midday stem water potential as a means to directly quantify tree water status and understand the extent of tree stress that is being imposed by deficit irrigation.

Once an orchard has been designated for deficit irrigation and a pressure chamber is available, withholding water during late season tree and nut growth stages will most likely save the most water with the least negative impact on tree health and productivity. As little as one inch of applied water postharvest can minimize the chance for autumn frost damage. Sufficient winter rains may follow during the dormant period and further address winter die-back concerns. In the absence of winter rainfall, dormant season irrigation remains an option at a time when water may be more available since some annual croplands may be fallow or crops like alfalfa may be dormant.

If more water savings are needed, withholding water during mid season kernel fill and nut development stages may be effective. A mild water deficit during mid season coupled with late season stress can save up to about 12 inches of applied water. It is likely that yield will be reduced and some crop quality parameters will be affected, but impacts on canopy development can be reduced and the time required to return to full production after the water shortage has passed may be minimized.

If existing water supplies are limited such that even more water savings are needed, regulated deficit irrigation can be imposed with greater intensity and may save up to 20 inches of applied water per acre, but as discussed earlier, the effects on the orchard can be dramatic. Under this scenario, it may be necessary to consider other management measures beyond regulated deficit irrigation.

COMPLEMENTARY ACTIONS.

Other management actions should be considered along with regulated deficit irrigation. They include:

Utilize well designed and maintained irrigation systems that apply water efficiently.

To minimize water use, it is helpful to control weed growth and avoid planting cover crops in orchard drive rows. Cover crops, depending upon the coverage and time of season, can increase orchard water use by as much as 20 to 30 percent and reduce the amount of rainfall that is stored during dormancy.

Pruning practices may require further consideration depending upon the extent of the water shortage. Trees will not be as responsive to hedging or pruning when water is restricted and it may not make as much sense to prune bearing wood that might otherwise produce walnuts. In a worse-case severe water shortage scenario, where tree survival is of concern and production is not an immediate concern, heavy pruning may have a role to reduce the canopy size and subsequent water consumption.

In young orchards, try to avoid wetting portions of the orchard where there are no roots present to take up water. Irrigating areas without roots can result in increased evaporative losses, promotion of weed growth, and wasting limited water supplies. Placing soil moisture sensors, such as Watermark sensors, directly in the root zone of young trees can be helpful (Lampinen et al., 2008b). If the orchard is flood irrigated, try applying water in only 2 furrows, one on each side of the tree row. This can also be done in an orchard which will eventually be solid-set sprinkler irrigated. Irrigating with only 2 furrows per tree row in a young orchard may result in less water use than wetting the entire orchard floor using solid-set sprinklers. Solid-set sprinkler or micro-sprinkler irrigated orchards can also be irrigated with drip irrigation to target irrigation water at the root zone during orchard establishment. Depending on the water quality, filtration may be required to keep the drip system from clogging. Be certain that the irrigation system design accounts for the irrigation system which will ultimately be used to irrigate the mature orchard.

BEYOND REGULATED DEFICIT IRRIGATION.

Depending on degree of the water shortage, there will likely become an economic breakpoint where regulated deficit irrigation is no longer sufficient to cope with water shortages in walnut due to its cumulative impact on tree growth and production potential. When that point is approached, additional investment in water supply such as well construction to provide groundwater to augment existing water supplies may be necessary. The economic loss from prolonged deficit irrigation of walnut can be significant and may warrant such investment. If investing in supplemental water supplies is not possible, then it may be necessary to acknowledge that walnut production is not a suitable match for the available water resources.